Eagle aircraft cameras are entirely automatic in action, and the type III, which
takes photographs measuring 5 inches square, can be used either for oblique
photography or vertical-survey work. In this case it was decided to fix the
camera in the floor of the aircraft for vertical photography only, suspended on
an insulated gimbal mounting.

A sectional drawing of the camera is shown on this page. It is constructed
entirely of metal, chiefly aluminium and duralumin, and the mechanism, which is
intermittent in action, is driven by an electric motor through a flexible shaft
and worm gearing to the main gear-wheel shown at A.

The magazine B is detachable and is driven through the vertical bevel shaft C.
Sufficient film for 125 exposures is carried in each magazine and along the edge
of each exposure photographic records are made simultaneously of instruments
carried in the receptacle in the side of the camera. On this expedition the
instruments consisted of
(1) a veeder counter for serially numbering the photographs,
(2) a watch with centre second hand to indicate the time and number of seconds
between each exposure,
(3) an ivorine tablet giving the date and subject to be photographed, and
(4) an aneroid. With regard to (4), since great variations in altitude were to
be included in the photographic strip,

Two interchangeable instruments were supplied in order to cover the complete
range with as open a scale as possible. The first instrument registered from 0
to 17,000 and the second, which had to be changed in Right, from 15,000 to
33,000 ft. Each instrument is photographed through a separate lens, and
illuminated by small electric globes, the actual duration of exposure being
regulated by the pneumatic switch F. The 5-inch square ground photograph is
taken by a special 5-inch focus Ross Xpres lens working at an aperture of f/4.
The semi-angle is thus 35° and fully illuminates the picture with perfect
definition.

Immediately behind the lens is the all-metal Louvre shutter G, a comparatively
recent invention but one which has made such a survey flight as this possible
and the perfection of the results a certainty. This shutter consists of a number
of thin pivoted metal blades which open and close at extremely high speed,
exposing the whole of the aperture at once with a minimum loss of light and with
great efficiency. In view of the height (approximately 30,000 ft.) at which
exposures had to be made, the chief precautions to be taken were against

(1) low atmospheric pressure and
(2) extreme cold.

Apart from the fact that the operator, besides having other duties to perform
during the flight, could not expect to be able to do more than switch on the
camera and regulate the intervals between exposures, the only part of the
mechanism likely to be affected by the rarefied atmosphere was the pneumatic
instrument switch. Various experiments in a specially constructed vacuum chamber
with electric-operating solenoids showed that a definite and too pronounced
shortening of exposure of the instrument occurred half-way up the altitude
scale, and in consequence a two-way switch was fitted which had to be operated
.when the high-reading aneroid was substituted for the low.

With regard to the drop in temperature it was thought originally that since the
camera mechanism would function quite well without oil for the short period for
which it would be required, suitable allowance in the fitting of spindles and
bearings, and thermostatically-controlled electric heaters for the film chamber
only, would overcome the difficulty. It was eventually. decided, however, that
temperatures in the neighbourhood of - 50° C. might be encountered and the
problem was, therefore, further complicated.

Luckily, it was found at this stage that a further supply of electric current
was available from the aircraft generator for heating purposes, and internal
heating elements and external electric blankets had to be made. Messrs. Hall and
Co., of Deptford, most generously came to our aid by preparing and maintaining
day and night, special cold chambers by means of which the actual conditions
were reproduced and eventually overcome.

Strangely enough, the most delicate part of the whole photographic equipment,
the electric control or intervalometer, which determined and regulated the time
interval between exposures, was found to function with the utmost reliability
after only minor alterations to the magnetic field. As previously stated, a
5-inch lens was selected with an extremely .wide angle of view, which meant that
a ground area of 4 square miles could be covered by each photograph at the
average height above the ground at which it was arranged to fly. A calculation
of the total area to be surveyed showed that two magazines each containing 125
exposures would suffice for the journey out to the mountain and back and provide
overlapping pictures. Since a study of the contour map of the district, so far
as is known, reveals vast and precipitous changes in ground level, a
stereoscopic inspection of the photographs obtained should prove of immense
interest.

Both aircraft were similarly equipped, and, in addition to the survey camera,
carried two cameras for oblique pictorial photography, viz., a Williamson P14
taking 5 inch x 4 inch plates, and a small standard pistol camera taking 3.5
inch x 2.5 inch plates. The fact that the latter was used without any heating
arrangements or special preparation and was, in fact, delivered " off the shelf
" at the last moment, speaks volumes for the value of Louvre shutters.

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